Expert Witness Methodology & Principles the “Key” to Admissibility


In Daubert, the court underlines that admissibility should be based only on an examination of the expert’s principles and methodologies and not on the conclusions that they arrive. That is, whether those principles and methodologies can be challenged in some objective sense, or whether it is instead simply a subjective, conclusory approach that cannot effectively be evaluated for reliability. According to Daubert, an expert’s opinion must derive from sufficient data, utilizes reliable principles and methodologies and it may be able to be reproduced by independent testing. Meaning that, if another expert cannot reproduce that expert witness’s principles and methodologies where they based their opinion, the expert witness is unlikely to survive a Daubert challenge.

Related Articles: The Acceptable “Rate of Error” in Expert Witnesses’ Opinion

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Inadequate Warnings About Side Adverse Effects

For example in a recent products liability California case, Hexum v. Eli Lilly & Co., 2015 U.S. Dist. LEXIS 106153 (C.D. Cal. Aug. 10, 2015) regarding inadequate warnings about side adverse effects of a medical treatment “Cymbalta”, the court excluded the plaintiff’s two experts from testifying about opinions for failing to cite any relevant authority supporting their respective opinions. In general, the court held that the opinion of the first expert witness, a psychiatrist, regarding the severity of discontinuation symptoms were inadmissible because the expert invented his own definitions and rendered his expert opinion of inadequacy of Cymbalta label on that basis since the only authority he cited to arrive to his conclusion was his own book, cited at glance without further references. In addition, the court determined that the first expert was not qualified to render an opinion regarding plaintiff’s need for medical treatment based on fibromyalgia and her possible reaction to treatments other than the one at issue since fibromyalgia was not his field of expertise, he never published and treated anything in that medical area.

In the same way, the court held that the opinion of the second expert witness, a social psychologist, regarding a reader’s interpretation of Cymbalta’s labels based on semantic and linguistic interpretation was inadmissible to the extent that he was not offered as an expert in the English language or grammar; and thus, the expert did not base his conclusions on reliable methods instead he relied on his own examination for the label’s particular language and how the information was communicated. Thus, the court held, because the expert’s opinion did not rely on any particular expertise, it was not helpful to the jury; and thus, he was unqualified to render his opinion based on his own experience since it was not the result of reliable methods.

Possible questions for testing:


What theory or method did you base your opinion on?

How did you test this theory or method?

What were the results?

How many times have you used this theory or method?

Have you always used the same method when you used this theory?

Have you ever had different results or results inconsistent with the theory or method you based today’s opinion on?


Does any part of the theory or method require subjective interpretation?

If yes, describe what aspect of the testing is subjective.

If yes, do you believe another expert might interpret the same method and data differently?

Is there any other method you use to evaluate the subjective part of this theory?

Did you use that method in concluding at your opinion in this case?

Related Articles: All Daubert Articles